This invention relates to a method and an apparatus for measuring a dynamic balance of a tested member, capable of measuring the imbalance to be removed of a tested member which should have a predetermined dynamic imbalance remaining therein.
That is, in case a dynamic balance of a crank shaft for an engine of an automobile, for example, is to be measured, it must be taken into account that, when a crank shaft is rotated with being built in an automobile, several additional members such as a piston, a connecting rod and so on are mounted thereto.
In other words, it is meaningless to measure a dynamic imbalance of a tested crank shaft itself. It should rather be measured whether there is the imbalance to be removed of a tested crank shaft for allowing the crank shaft to be rotated without vibrating when the related members are mounted thereto.
In order to measure such an imbalance of a tested member as described above, there have been employed the following three methods.
In a first method, a plurality of dummy rings corresponding to the imbalance to remain are attached to respective portions of the crank shaft, when rotated to determine correction amounts and angle positions therefor.
In a second method, a crank shaft is connected on a vibration stand to a spindle which carries a dummy weight equivalent to the predetermined imbalance to remain, when rotated to determine correction amounts and angle positions therefor.
In a third method, a crank shaft itself is rotated without attaching any dummy ring or dummy weight, and the imbalance of the rotating crank shaft is detected by a vibration sensor as an analogue signal which is then converted to digital signal by an A/D converter (Analogue-Digital converter). Thereafter, a numerical calculation is conducted based on the detected digital value and a digital dummy value obtained in advance and corresponding to the imbalance to remain, i.e., the latter is subtracted from the former, thereby finding an initial imbalance and deciding correction amounts and angle positions therefor.
It should be noted here that the balance adjustment of the crank shaft after the above described measurement based on the above described correction amounts and angular positions is performed by making a drill hole at the optimum position with the optimum depth on the counter weight of the crank shaft.
However, the above described first method requires attaching and detaching of dummy rings for each crank shaft to be measured. Further, when crank shafts of various types are to be measured by the same measuring apparatus, different dummy rings for different types have to be respectively prepared and replaced each time. Moreover, for these reasons, it is difficult to automate the measurements with this method.
Although the second method does not require attaching or detaching of dummy rings for respective crank shafts, it requires replacing dummy weights when different type crank shafts are to be measured by the same measuring machine.
The third method requires neither attaching nor replacing of dummy rings or dummy weights so that it is suitable for automation of the measurements, but is hard to attain measurements with high accuracy because of the following reasons.
That is, the imbalance data obtained by a vibration sensor is the sum of the imbalance to remain and that to be removed as described above. Although the imbalance to remain is different depending on the type of a crank shaft, some reaches a level of several thousand gcm. In contrast, the imbalance to be removed is rather smaller, i.e., generally of a level of several hundred gcm.
Accordingly, the maximum size of the imbalance data obtained by the vibration sensor comes to several thousand and several hundred gem.
On the other hand, the voltage which can be inputted to the A/D converter is usually limited to below xc2x15V, and the full scale of the input range of the A/D converter should be made to correspond to the possible maximum value of the imbalance data measured by the above sensor.
By doing so, however, the imbalance measurable per one digit by the A/D converter becomes larger (i.e., the resolution of the A/D converter becomes low) as the above maximum value becomes larger and, hence, the measurement with the resolution higher than the above becomes impossible. That is, the resolution becomes worse due to the presence of the relatively large imbalance to remain, and therefore the measurement with high accuracy for the imbalance to be removed becomes difficult.
It is therefore an object of the present invention to provide a method and an apparatus for measuring a rotational balance of a tested member which should have a predetermined imbalance remaining therein, capable of accurately detecting the imbalance to be removed, without requiring a dummy ring, a dummy weight or the like.
The measuring method of the present invention is such that the digital dummy value information equivalent to a dummy portion is stored in a memory in advance and a rotation sensor detects the rotational angular position of the body of rotation as a workpiece and, in response to this rotational angular position, digital dummy value information is read out from the memory, accompanied by which these pieces of information are converted to an analogue dummy signal and then the analogue vibration signal obtained by detecting the rotational vibration amount of the body of rotation by a rotational vibration sensor and the analogue dummy signal are synthesized so as to obtain the analogue signal of the initial imbalance portion by removing the dummy portion from the imbalance portion, after which this analogue signal is digital-converted so as to obtain the digital initial imbalance value.
The above described digital dummy information is desirably stored in a memory for each different type of the apparatus and which is, every time the type of the workpiece changes, made to switch over to the dummy value information corresponding to the type changed. In this manner, an automatic measurement becomes possible while the workpieces of different types may follow on the same measuring apparatus. Morever, when the initial analogue imbalance signal is digitalized, it is desirable that the approximate maximum value of the initial analogue signal is made to correspond to the full scale measurable by the A/D converter. Also, the analogue signals are preferably divided into components orthogonal to each vibration direction, which are generated by the cosine waveform and the sine waveform completely synchronized with the body of rotation, respectively.
The rotational balance measuring apparatus of the present invention is provided with a rotational support portion for supporting the body of rotation as a work, which is provided with an electrical motor capable of rotatingly driving this body of rotation. Also, a rotation sensor for detecting the rotational angular position of the body of rotation is provided and a vibration sensor for obtaining an analogue vibration signal by detecting the vibration of the above described body of rotation through the rotational support portions provided. On the other hand, a memory for storing the dummy value information as a digital data of the imbalance to be left over in the body of rotation is provided and a controller for outputting the above described dummy value information based on the rotational angular position signal from the above described rotation sensor is provided. With this outputted digital dummy value information inputted, a D/A converter for converting this information to an analogue dummy signal is installed and a wave synthesizing circuit is installed where the analogue dummy signal obtained here and the analogue vibration signal from the above described rotation sensor are inputted and wave-synthesized so as to obtain an analogue initial imbalance signal by removing the analogue dummy signal portion from the analogue vibration signal. To this wave synthesizing circuit, a A/D converter is connected where the analogue initial imbalance signal is inputted and converted to a digital imbalance signal.
The above described D/A converter is preferably configured by a D/A converter for producing a wave of one directional component of the analogue dummy signals based on the COS waveform and a D/A converter for producing a waveform of the component orthogonal to the above described one direction of the analogue dummy signals based on the SIN waveform for each of the vibrational directions orthogonal to each other.
Morever, it is preferable that the above described dummy information is made to correspond to the result obtained by measuring by a master workpiece as the predetermined imbalance to be left over in advance and at least the size and the angular position of the imbalance at a measured rotational speed of the body of rotation are stored in a memory in terms of a digital value. That is, the master work is rotated in a state of being attached with a dummy weight equivalent to the predetermined imbalance portion of this master work and is corrected and balanced by measuring by a rotational balance measuring device. After this, the analogue signal obtained by the vibration sensor by measuring the rotational balance of the master work piece with the dummy weight is digital-converted and taken as the data of the digital dummy value. In order to obtain the dummy value information by this master work piece, it is desirable to use the rotational balance apparatus similar to the one used for measuring the work piece as hereinafter described.
Also, the waveform synthesizing circuit may be preferably capable of complying with either a waveform addition circuit or a waveform subtraction circuit by changing the plus and minus of the wave to be inputted, in short, capable of obtaining the analogue initial imbalance signal by removing the analogue dummy signal portion from the above described vibration signal.
Morever, the rotational support portion of the rotational balance apparatus may be configured preferably by a bearing for supporting the outer periphery of both ends of the body of rotation or by a spindle for supporting both ends of the rotation axis along the rotation center axis of the body of rotation.
The measuring method and the measuring apparatus of the present invention are configured as described above and therefore have the following effects.
For example, supposing that the size of the dummy portion is 4,500 gcm and the size of the initial imbalance portion which has to be adjusted in the crank shaft itself as a result of deviation from the dummy portion is 500 gcm, the size of the detection value detected by the vibration sensor is a sum of both of the above values which is 5,000 gcm.
When the measurement is made by the above described conventional third method, the full scale of the measurement range in the A/D converter has to be made to correspond to at least the above described sum which is 5,000 gcm.
If the full scale of the A/D converter is below xc2x15 and its resolution is 12 bits, the above described conventional third method will obtain the accuracy of 5,000 gcm+2048 digits+2.44 gcm/digit only and the measurement of a level smaller than this becomes impossible.
In contrast, according to the measuring method and the measuring apparatus of the present invention, 4,500 gcm of the above described dummy portion are taken as the analogue signal of the initial imbalance portion only where by addition or subtraction of analogue signals the dummy portion is removed from the analogue vibration signal of the vibration sensor. That is, the size of the signals inputted to the A/D converter is equivalent to a maximum of 500 gcm only. Accordingly, if this is digital-converted by the above described A/D converter, 500 gcmxc2x12048 digits=0.244 gcm/digit and, according to the measuring method and the measuring apparatus of the present invention, ten times the resolution is obtained in contrast to the above described third method and the measurement accuracy is greatly improved. Consequently, even in the case where the amount of the predetermined imbalance is large, the initial rotational imbalance can be made far smaller than that of the conventional technology.